Agriculture Reference
In-Depth Information
to result in more uniform descriptions than in the earlier periods of soil survey (Simonson, 1963).
One manifestation of this improvement in the detail and precision of soil descriptions was the
development and adoption of the standard form (SCS-SOI-232) for recording soil descriptions.
Although this was ofÝcially a form used within the Soil Conservation Service, it was commonly
used or adapted by other NCSS partners. These standard Ýeld description templates had the
beneÝcial effect of reminding soil scientists to record many of the important properties of the soil,
such as moist and dry color, percentage of coarse fragments, mottles, ped coatings, pH, horizon
boundaries, etc.; and to identify diagnostic horizons and features that are present. However, Dr.
Simonson (then Director, Soil ClassiÝcation and Correlation in the Soil Conservation Service)
expressed the concern that while it tended to set a standard for the minimum set of data to be
recorded, it may also have had the unintended consequence of setting the maximum (Simonson,
1987).
Another important way that Soil Taxonomy has improved the soil correlation process is by
grouping soils with similar properties into classes, thereby making it easier to understand how they
relate to one another. The number of soil series recognized in the United States has grown from
about 10,500 in 1975 to nearly 22,000 today, clearly too many for the human mind to organize
and remember. With the aid of the classiÝcation to group the soils with similar properties, and
computer technology (Soil Survey Staff, 2001c) to query our soil database and quickly deliver
series descriptions for comparison, it is fairly easy for todayÔs soil scientists to Ýnd the names of
all the soil series in a given class, or closely similar class, and to coordinate the naming and
interpreting of soil map units from one survey project to another.
Some Problems with Quantitative Limits
The use of quantitative class limits has also had some drawbacks. Webster (1968) expressed
several objections regarding the system. A major problem with the approach is that while the class
limits are Ýxed, there is inherent uncertainty in the observation and measurement of many of the
properties, such as recording color by comparison to color charts, or determining rupture resistance
class by attempting to crush peds in the Ýeld. Laboratory measurement of properties such as particle
size, base saturation percentage, or organic matter content also contain inherent measurement error.
As a result of these uncertainties, data obtained by repeating a Ýeld observation or laboratory
analysis could result in a different classiÝcation for the soil. Also, with no provision for overlapping
of class limits, some soil bodies that have natural distributions of one or more properties that
straddle artiÝcial class boundaries will be forced into separate taxa, even though they clearly form
a natural cluster in their landscape setting. As Soil Taxonomy has evolved, it has become increas-
ingly dependent on the need for laboratory data for supplying the required quantitative values that
successfully classify a soil (e.g., spodic materials, andic soil properties, cation exchange activity
classes, etc.). While this reliance on laboratory measurements can be a positive addition to the
quality of the data in the soil survey, obtaining it can be time-consuming and expensive, thus
hindering our ability to classify pedons with conÝdence at the time they are being observed. Also,
it is an impediment to the effective use of Soil Taxonomy in places where analytical laboratory
services are not readily available.
TAXONOMIC UNITS AND MAP UNITS
Cline (1977, p. 253), in an attempt to anticipate future developments in soil survey, wrote
At the lowest level of the system, we will have to acknowledge the differences between taxonomic
soil series and mapping units that bear the same name and will probably have to rectify the confusion
